Conventionally, phthalocyanine compounds are very useful as pigments in the field of coloring material industries, and many investigations have been conducted on such compounds for a long time. Phthalocyanine pigments can exhibit vivid color tone and high coloring (tinctorial) power, and they are widely used as cyan colorants in many fields. Examples of use applications in which the pigments are used include paints, printing inks, electrophotographic toners, ink-jet inks, and color filters. The pigments are important compounds indispensable in everyday life at the present time. Practically particularly important applications of phthalocyanine pigments as color materials (colorants), which need to have high performance in particular, include inkjet inks and color filters.
As the coloring material for ink-jet ink, dyes have been used, but they have drawbacks as to water resistance and light resistance. To overcome the drawbacks, pigments have come to be used. As cyan pigments, use may be mainly made of copper phthalocyanine pigments. Images obtained from pigment inks have remarkable advantages of superior light resistance and water resistance compared with images obtained from dye-based inks. However, the former images have the problems that the pigment is not easily formed uniformly or pulverized into fine-particles of a nanometer size, which can permeate pores in the surface of paper, and then the pigment in the image are poor in contact or adherence property to the paper.
With an increase in the number of pixels in digital cameras, there is a need for a color filter used in a CCD sensor to be made thinner. Some color filters use organic pigments (including metal phthalocyanine compounds as cyan pigments). Since the thickness of the filter depends largely on the particle diameter of the organic pigment, there has been a need to produce stable fine-particles of a nanometer-size level.
In some fields other than the field of coloring material industries, phthalocyanine compounds are used in such fields in which semiconductivity or photoconductivity of the compounds are utilized. For example, investigations have been conducted on electrophotographic photoconductors or laser printer photoconductors, based on the photoconductivity of metal-free phthalocyanines, or a variety of metal phthalocyanines, such as copper phthalocyanine, vanadyl oxyphthalocyanine, aluminum chlorophthalocyanine, zinc phthalocyanine, hydroxygallium phthalocyanine, and titanyl phthalocyanine.
Some types of metal phthalocyanines have redox power, and thus attention has focused on their application to catalysts. Since phthalocyanine compounds have multiple functions as mentioned above, not only non-metallic or copper phthalocyanines but also various types of metal phthalocyanines are increasing in importance (see “Pigment Dispersion and Stabilization and Surface Treatment Techniques and Evaluation,” 2001, pp. 123-224, published by Technical Information Institute Co., Ltd., Japan; Masato Tanaka and Shouji Koma, “Phthalocyanines: Their Basic Physical Properties and Application to Functional Materials,” 1991, pp. 55-124, published by Bun-Shin, Japan).
Examples of methods for producing metal phthalocyanines (e.g. copper phthalocyanine) include a method by reaction of phthalonitrile with a copper salt; a method by reaction between phthalic anhydride, a copper salt, urea, and ammonium molybdate; and a method by reaction of phthalonitrile with a copper salt in the presence of a strong organic base (W. Herbst and K. Hunger, “Industrial Organic Pigments, Production, Properties, Applications; Second Completely Revised Edition,” VCH A Wiley Company, 1997, pp. 595-630).
Pigments of copper or any other transition metal phthalocyanines are hardly soluble in common solvents. Thus, it is not easy to produce a high-purity pigment, by removing by-product impurities that result from the above method. Disclosed examples of methods for producing high-purity transition metal phthalocyanines include (1) acid-paste methods, and (2) indirect synthesis methods using alkali metal phthalocyanines.
Acid-paste methods comprise the steps of: dissolving a crude reaction product in a strong acid (generally concentrated sulfuric acid), with the benefit of relatively high solubility of copper phthalocyanine or the like in a strong acid; and pouring the solution into ice water, to precipitate particles. This method can control particle size and provide a relatively high-purity product. However, the acid used in this method is highly oxidative, and thus it can produce new decomposition impurities, which can often degrade the performance of the product for use in electronic materials, catalysts, or the like, although their amount is very small.
The indirect synthesis methods using alkali metal phthalocyanines comprise the steps of: first, synthesizing an alkali metal phthalocyanine that is highly pure and relatively easy to dissolve in an organic solvent; dissolving or dispersing it in an organic solvent; and allowing it to react with a copper salt or any other transition metal salt dissolved or dispersed in an organic solvent, to precipitate a transition metal phthalocyanine. Disclosed examples of this method use either dilithium phthalocyanine or dipotassium phthalocyanine. These methods are further described in below.
Metal-free phthalocyanines are also hardly soluble compounds in organic solvents, although they have slightly better solubility in organic solvents than copper or any other transition metal phthalocyanines. When alkali metal phthalocyanines are brought into contact with a solvent with high acidity (which seems to have pKa<˜17), such as water and alcohols, they can take a proton from such a protic solvent, to form precipitates of hardly soluble metal-free phthalocyanines. Among the alkali metal phthalocyanines, however, dilithium phthalocyanine is relatively stable and soluble in absolute ethanol. In 1938, Barrett et al. reported that, based on such properties, dilithium phthalocyanine can be used in the synthesis of various transition metal phthalocyanines, through reaction with transition metal salts in absolute ethanol (P. A. Barrett, D. A. Frye, and R. P. Linstead; J. Chem. Soc., 1938, 1157).
P. A. Barrett, D. A. Frye, and R. P. Linstead (supra) also reported that dilithium phthalocyanine was “freely” soluble in ethanol. Actually, however, dilithium phthalocyanine is not very soluble, and the reaction does not proceed in a uniform solution; rather, it converts dilithium phthalocyanine dispersed in a solution into another dispersed metal phthalocyanine. In such a process, therefore, it is difficult to control the precipitation and particle size. Concerning stability, the reaction of dilithium phthalocyanine with alcohol can be suppressed when it rapidly reacts with transition metal ions in the presence of a transition metal salt. If the reaction time becomes longer because of scaling up or the like, however, the possibility of generating metal-free phthalocyanine by-products can increase.
In alcohols, dipotassium phthalocyanine is rapidly converted into metal-free phthalocyanine. In alcohols, therefore, it is not possible to perform the reaction between dipotassium phthalocyanine and a transition metal salt. In 1986, Kinoshita et al. disclosed a method in which dipotassium phthalocyanine is allowed to react with a transition metal salt in a hydroxyl-free organic solvent (JP-A-61-190562, “JP-A” means unexamined published Japanese patent application). In 1980, Wolfgang et al. disclosed a method of purifying metal-free phthalocyanine, which comprises the step of: heating dipotassium phthalocyanine together with an ether-series solvent, such as a crown ether or diethyleneglycol dimethyl ether (hereinafter referred to as “diglyme”), dimethyl sulfoxide, and dimethylformamide, so as to form a soluble complex (U.S. Pat. No. 4,197,242). Based on the disclosed method using diglyme, Kinoshita et al. found a method of synthesizing a metal phthalocyanine, which comprises the steps of: synthesizing a solution of a dipotassium phthalocyanine bis(methoxyethyl)ether complex, and allowing it to react with a transition metal salt. In this method, dipotassium phthalocyanine is uniformly dissolved in diglyme, in contrast to the case of dilithium phthalocyanine, but the transition metal salt is dispersed in diglyme during the reaction. Such a method is not sufficient for particle-size control in the synthesis of metal phthalocyanines.
In general, the methods to produce fine particles of a pigment are roughly classified into a breakdown method, in which fine particles are produced from a bulk material by pulverization or the like, and a build-up method, in which fine particles are produced by particle-growth from a gas phase or liquid phase, as described, for example, in “Experimental Chemical Lecture, 4th Edition,” edited by the Chemical Society of Japan, vol. 12, pp. 411-488, Maruzen Co., Ltd., Japan. The pulverizing method, which has been widely used hitherto, is a fine-particle-producing method having high practicability, but it has various problems, such as that its productivity is very low in producing organic material particles of nanometer size, and that the materials to which the method can be applied are limited. In recent years, investigations have been made to produce organic material fine-particles of nanometer size by a build-up method.
In a build-up method, fine particles can be produced in a more stable manner, by allowing a dispersing agent to coexist at any time when a solution in which an organic pigment is dissolved is gradually brought into contact with an aqueous medium serving as a poor solvent for it, so that the pigment is precipitated (a so-called coprecipitation method ((a reprecipitation method)), JP-A-2003-26972). This method is effective as an easy method for producing nanometer-size particles. Under the present circumstances, however, this method cannot be applied to the synthesis of fine particles of copper or any other transition metal phthalocyanine pigments. Because for use of this method, there has been found no acceptable method to construct a copper phthalocyanine-production system that satisfies the prerequisite that the organic pigment or its precursor should uniformly be dissolved in a solvent, and that, if a reactant is present in a poor solvent, the reactant also should uniformly be dissolved in the poor solvent. Thus, there has been a demand for proposal of a system that allows the synthesis of fine particles of such a pigment as copper phthalocyanine by the reprecipitation method.